The invention relates to a method for inductive distance measurement.
Method for inductive measurement of a distance between a workpiece and a working head are already generally known. The working head in this case bears an induction coil, through which an alternating current flows and which is part of a resonance circuit whose oscillation frequency is monitored for changes which occur as a consequence of changes in the distance between the workpiece and working head.
Moreover, working heads for workpiece machining are also generally known which have an outlet channel which faces the workpiece and is surrounded concentrically by an induction coil, in order to determine the distance between the workpiece and working head inductively with the aid of the coil.
It is the object of the invention to develop a further field of application for the method mentioned above. Likewise, a further working head suitable for carrying out this method is to be made available.
A method according to the invention for inductive measurement of a distance between a workpiece and a working head which bears an induction coil through which an alternating current flows and which is part of a resonance circuit whose oscillation frequency is monitored for changes which occur as a consequence of changes in the distance between the workpiece and working head is distinguished in that the working head acts on the workpiece with a material having a dielectric constant greater than 1; the frequency of the alternating current is in the megahertz region or just therebelow; and the induction coil is screened from external electric fields.
By contrast, a working head for machining a workpiece which has an outlet channel facing the workpiece and which is surrounded concentrically by an induction coil is distinguished in that a material with a dielectric constant greater than 1 can be guided through the outlet channel, and the induction coil bears an electrical screen.
It is possible with the aid of the invention to determine the distance between the working head and the workpiece exactly in an inductive way even when the workpiece is acted on by the working head with a medium whose dielectric constant ∈ is greater than or very much greater than 1. For example, the medium acting on the workpiece can be water, that is to say a water jet with or without abrasive material, or a flowable adhesive which is to be applied to the surface of the workpiece.
In this case, the method and working head must be capable of carrying out the respective process steps even in very narrow spaces of, for example, three-dimensional workpieces, with the result that as compact an arrangement as possible is desired. Consequently, the aim is to select the frequency of the alternating current flowing through the induction coil to be relatively high, and this favors selection, in particular, of relatively small electric components. On the other hand, however, because of the relatively high dielectric constant of the material to be processed, in the case of high frequencies relatively high parasitic capacitances occur which can falsify the distance measurements. Consequently, according to the invention it is also proposed to screen the induction coil from external electric fields in order to avoid the influence of parasitic capacitances.
Because of the selected and relatively high frequency of the alternating current flowing through the induction coil, the depth of penetration of the current induced in the workpiece is only slight, with the result that the method operates essentially independently of the thickness of the workpiece. Moreover, there is the further advantage that the method is also largely independent of the type of material of the workpiece to be machined.
In a refinement of the invention, the frequency of the alternating current can be in the region from approximately 500 kHz to approximately 20 MHz, preferably in the region from three to eleven MHz.
For the purpose of electrical screening of the induction coil, the latter can, for example, be screened by a screening conductor, but also by an electrical screen which is such as to surround on the outside a coil packet formed by the induction coil. The screening can thus be toroidal or in the shape of an annular box. Here, in the case of a metallic working head the electric screening of the coil packet can also be completed by that wall region on which the coil packet is seated.
In a still further refinement of the invention, an LC resonance circuit containing the induction coil is arranged on the induction coil or in the vicinity thereof on the working head, and this improves the measurement results, since in this case external influences, for example temperature fluctuations, act in the same way on all parts of the resonance circuit.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.